July 8, 2011

[* This title recycles an item from World Climate Report from November, 1995 (yes, we have been at this for a while). To wit:

“In an article intended to raise public consciousness about the threat of global warming, environmental writer Bill McKibben mentioned reports that, in northern Russia, “venomous snakes had appeared for the first time…” Well, venomous snakes will certainly get people’s attention. (This is reminiscent of the old college poster ploy—SEX SEX SEX! Now that I have your attention, my Western Civ textbook is for sale.)

Much to our dismay, McKibben never did expand on the global warming/snake connection…”]

Back in the end of May, we ran a piece titled “No Long-term Trend in Atlantic Hurricane Numbers” that described the results of research conducted by a team of researchers made up of Gabriele Villarini, Gabriel Vecchi, Thomas Knutson, and James Smith. The Villarini gang examined the observed Atlantic hurricane record and determined that the apparent upward trend in the annual number of hurricanes observed between 1878 and 2008 was being driven by an increase over time in the number of “shorties”—that is, hurricanes which lasted two or fewer days in duration. And, they determined that the increasing number of shorties was an artifact of the changing observational systems that had been in place over the years, rather than an actual secular change in the true number of events. In other words, after accounting for changes in observing practices and technologies, there was no long-term trend in Atlantic hurricane numbers (a result that is hard to blame global warming).

That finding was just another in a long string of similar papers that had been published in the scientific literature in recent years and covered here at World Climate Report on the topic of global warming and tropical cyclones.

Now, yet another paper has come to our attention that concludes that there has been no long-term trend in Atlantic hurricane numbers. Since readers may simply skip over our summary of this new paper, with a “yeah, yeah, I know that already” if we titled our piece something like “No Long-term Trend in Atlantic Hurricane Numbers—Even More Evidence”, we decided to spiced up the title a bit to try to grab your attention. If you’ve read this far, it must have worked, and we should apologize for the content to come—there is nothing sexy involved, just more evidence, as you may have guessed by now, that there has not been a long-term trend in Atlantic hurricane numbers. Sorry.

In paper published in the Journal of Climate, Gabriel Vecchi and Thomas Knutson (a subset of the authors of the “shorties” paper describe above) reached essentially the same conclusion as Villarini et al., albeit using a different analysis technique.

Prior to the satellite era (i.e., before 1966), most of the information about tropical cyclones in the Atlantic ocean came from either ship tracks (most of which tried to avoid the storms) or when the storms passed close enough to be detected from land. This tended to leave a lot of the Atlantic Ocean basin unmonitored. And in these observation-free areas, hurricanes may have come and gone without being detected. Using information about historical ship tracks, Vecchi and Soden determined the time and location of these unmonitored areas each year in the pre-satellite era, and from this information produced an estimate of the number of storms each year that may have been missed (based upon statistics of observed storms). They then add their estimate of the number of “missed” storms to the number of observed storms to produce an adjusted history of hurricanes in the Atlantic basin—one in which they think gives a more accurate picture of the temporal variation of hurricane occurrence.

Figure 1 shows their result. It shows the history of Atlantic hurricane counts as determined from the standard hurricane observation dataset as compiled by the National Hurricane Center (a set of observations referred to as the HURDAT dataset) along with the hurricane history as adjusted by Vecchi and Knutson.

Figure 1. Time series of Atlantic basin hurricane counts over the period 1878–2008 from (a) the unadjusted HURDAT and (b) after the adjustment for estimated missed hurricanes and the magnitude of the hurricane count adjustment. Plots show the annual (light lines) and 10yr running mean (dark lines) counts; (b) gray shading indicates the 95% method uncertainty on the adjustment. Dashed lines depict the linear least squares trends computed over the period 1878–2008. (from Vecchi and Knutson, 2011).

Notice that the upward trend in the original HURDAT dataset (top panel in Figure 1), becomes a (non-statistically significant) downward trend in the adjusted dataset (bottom panel in Figure 1).

Here is what Vecchi and Knutson have to say about this:

After adjusting for the estimate of missed hurricanes in the basin, the long-term (1878–2008) trend in hurricane counts changes from significantly positive to no significant change (with a nominally negative trend). The adjusted hurricane count record is more strongly connected to the difference between main development region (MDR) sea surface temperature (SST) and tropical-mean SST than with MDR SST. These results do not support the hypothesis that the warming of the tropical North Atlantic due to anthropogenic greenhouse gas emissions has caused Atlantic hurricane frequency to increase.

Pretty succinct.

It is worth noting that Vecchi and Knutson point out that Atlantic hurricane activity is more strongly related to the difference between the sea surface temperature in the main hurricane development region and the overall sea surface temperature across the tropics, rather than simply the sea surface temperature in the main development region itself. In other words, it is the relative temperature change that is more important that the absolute temperature change (a point that Vecchi and colleagues have been making for years). Therefore, the oft-heard reasoning that since global warming will lead to rising sea surface temperatures, that, voila, it will lead to a rising number of hurricanes, is ill-founded. The situation is much more complicated than that, and based on Vecchi’s earlier works, suggests that climate change resulting from an enhanced greenhouse effect will have little influence of hurricane activity. Look here for our summary of the team’s 2008 paper.

So there you have it, more evidence that there has been no long-term trend in Atlantic hurricane activity and that global warming has little role in the non-trend.